Thermal processing systems have been widely known and used for many years to perform a variety of semiconductor fabrication processes, including annealing, diffusion, oxidation, and chemical vapor deposition. As a result, these processes are well understood, especially with regard to the impact of process variables on the quality and uniformity of resulting products. Thermal processing furnaces typically employ either a horizontal-type furnace or a vertical-type furnace. For some applications, vertical-type furnaces are preferred because they create fewer particles during use, thus decreasing an incidence of contamination and workpiece waste. Furthermore, vertical-type furnaces can be easily automated, and typically require less floor space because of their relatively small footprint.
Conventional furnaces are typically designed to heat semiconductor wafers to desired temperatures either to promote diffusion of implanted dopants to a desired depth, or to perform other conventional processing techniques, such as an application of an oxide layer to a wafer or a deposition of a chemical vapor layer to the wafer. Heating requirements associated with the wafer are generally important and are typically closely monitored, especially when performing processes such as a spike anneal process.
Generally, a spike anneal process rapidly raises the temperature of the substrate from a relatively low temperature to a predetermined peak or target temperature, then cools the wafer as quickly as possible, thus minimizing a thermal budget associated with the wafer. The thermal budget, for example, is generally defined as the length of time wherein the wafer temperature is greater than a given temperature threshold. For example, a typical thermal budget temperature threshold can be approximately 50° C. less than the target temperature.
In order to achieve desirably low thermal budgets for spike anneal processes, hot wall thermal processing technology, for example, has been utilized. Hot wall thermal processing generally comprises moving the wafer upward and then downward in a bell jar furnace, thus exposing the wafer to a temperature gradient within the bell jar. However, controlling the wafer position in order to achieve a consistent spike peak temperature and a minimal thermal budget has typically been a problem.
Various temperature trajectories or profiles have been conventionally defined (both simple and time optimized), wherein full closed loop temperature control has been implemented for spike anneal-type thermal processing, and some success has been achieved. Temperature profiles can be generally reliably defined and utilized, provided that time-temperature curves generally define a non-aggressive spike profile (e.g., thermal budgets of 1.8 and higher), and that an average temperature ramp rate is generally sub-optimal.
In contrast, a Move-Wait-Move technique has also been utilized with some success. With the Move-Wait-Move method, the wafer is moved upward in the bell jar (e.g., moved to a higher temperature environment) under closed-loop position control using a predetermined position profile, rather than a temperature profile. The wafer, for example, moves from a pre-soak position that is associated with a relatively low temperature to the elevated position (e.g., the “Wait” position), wherein the wafer remains at the elevated position until a predetermined trigger temperature is reached. When the trigger temperature is reached, the wafer is moved downward (e.g., moved to a lower temperature environment), again under closed loop position control, following the predetermined position profile. If the trigger point is selected properly, the desired peak spike temperature can be achieved.
The Move-Wait-Move process can produce fairly aggressive thermal budgets with good repeatability for wafers when the trigger temperature has been correctly tuned. The Move-Wait-Move process produces several benefits over full closed loop processing. For example, critical temperature measurements are made while the elevator is stopped, as opposed to moving, such that artifacts produced by dynamic stray light compensation are eliminated. Furthermore, the motion which produces the final spike profile is performed using a simple point to point move, and as such, vibration for given velocity and acceleration limits are generally minimized. Still further, the motion profile, as well as the relationship between presoak height, presoak temperature and spike ramp height (e.g., the “wait” position), appear to be critical in the tuning and repeatability of radial uniformities from wafer to wafer. Consequently, variations in the actual move profile utilized in full closed loop processing appear to deleteriously increase the total range and maximum value of the radial thermal non-uniformity.
However, despite the advantages over full closed loop temperature control, it appears that the conventional Move-Wait-Move process (having a fixed predetermined presoak height, spike ramp height, and trigger temperature) cannot be effectively optimized for use with wafers of varying characteristics, such as wafers of varying emissivity, absorbtivity, or other radiative properties, since there is no dynamic compensation for such variations. Without some form of dynamic compensation for wafer variations, repeatability of wafer radial thermal uniformity is generally compromised. Consequently, in processing wafers having varying intra-wafer characteristics, as well as varying inter-wafer characteristics, control of radial thermal uniformity is quite difficult using conventional techniques.
Therefore, a need exists for a method for automatically varying the spike ramp height threshold used during the “Wait” part of the Move-Wait-Move spike technique, wherein the variation is based on dynamic measurements of wafer temperature. Such a method will improve the repeatability of both wafer-to-wafer thermal uniformity, as well as intra-wafer thermal uniformity, and will also optimize the thermal budget when applied to wafers of varying characteristics.